Sometimes energy efficiency is just a matter of practicality. As seems to be the case here: an art project that uses the wheels in a pair of Heelys to power a display on the top of the shoe.

The project, by New York artists Christian Croft and Kate Hartman, is called "Energy Harvesting Dérive." The display shows a series of random directions to the wearer, who, if they choose to follow them, will almost certainly get an off-the-beaten-path view of an area. The concept is derived from the 1960s dérive art movement, which advocated "technique or locomotion without a goal."

So if you can hack a pair of Heelys to power a microprocessor and tiny display, how about using it to recharge an iPod? Cell phone? Or, if you're really lost, GPS?

"Rather than reducing harnessed energy to the practical chore of battery charging, this work applies its energy towards a more playful application in hopes to promote discussion in the realm of sustainable energy development and alternative transportation design."